In general, according to one embodiment, a digital input signal is determined to be high when it has a voltage higher than a particular threshold, and determined low when it has a voltage below that threshold. For example, when a voltage of an input signal determined to be low rises and becomes above a threshold, it is determined that the input signal changed to be high. Similarly, when the voltage of the input signal determined to be high lowers and becomes below that threshold, it is determined that the input signal changed to be low.
In a circuit handling such digital signals, a Schmitt trigger circuit may be used. The Schmitt trigger circuit also receives a digital input signal, and outputs a digital signal according to the logical level of the input signal. In the Schmitt trigger circuit, a threshold above which the voltage of the input signal currently determined as low rises to be determined as having changed to be high differs from a threshold below which the voltage of the input signal currently determined as high lowers to be determined as having changed to be low.